Electric power cable lines are indispensable equipments to supply electric power energies to customers or subscribers, and the fault or damage of the equipments will result in significantly bad influence in the highly electrified societies. In the worst case, the functions of the societies will be paralyzed in almost all fields.
Therefore, it is the most important task that underground electric power lines installed in conduit or duct lines are well maintained and administrated to provide the stabilized state in the equipments. For this purpose, the inside of a manhole for installing and/or splicing electric power cables must be monitored.
In addition to underground electric power cable manholes, other manholes for gas supply pipes, water supply pipes, communication cable lines, etc. are required to be maintained and administrated for stabilized supply services of gas, water, communication, etc.
Conventional maintenance and administration in a manhole are carried out in a manner that a maintenance operator goes inside the manhole periodically or at the time of inspection of cables, etc. In such a case, following extremely troublesome operation and procedure are required.
(1) An application of obtaining an approval of operation on a road is required, because an opening of a manhole is mainly positioned on the road.
(2) A safety fence for an operator in operation on a road, a sign for indication of operation, an operator for control of traffics, an operator for safety-inspection of operation, etc. are required to be arranged.
(3) Auxiliary operation such as opening a lid, draining water, measuring a content of oxygen, etc. is required prior to entering a manhole.
(4) During operation, the measurement of oxygen content must be continued, and the ventilation of a manhole is kept to be carried by a blower.
In order to obviate such troublesome operation and procedure, some proposals have been explained in the Japanese Utility Model Kokai No. 3-44647, in which the first proposal is that an antenna is installed in a manhole closed by a lid to transmit a radio signal of, for instance, a bomb pressure via a key hole of the lid to a receiver position on the ground, the second proposal is that a lid for closing an opening of a manhole is used for an antenna, and the third proposal is that an antenna is protruded over a closed lid of a manhole.
However, the above proposals have disadvantages in that information of a manhole obtained on the ground is not precise, because an electric field strength of a radiated radio signal is weak, when some obstructing members of electric waves exist in the manhole. Especially, when a manhole has a double lid structure of outer and inner lids, electric waves are shielded by the inner lid. Consequently, information about equipment in the manhole is not obtained on the ground.
Accordingly, it is an object of the invention to provide a method and an apparatus for monitoring an inside of a manhole, in which no operator is required to go into the interior of a manhole.
It is a second object of the invention to provide a method and an apparatus for monitoring an inside of a manhole in which a radiated radio signal of a predetermined electric field strength is obtained on the ground.